Disordered-to-ordered transitions in assembly factors allow the Complex II catalytic subunit to switch binding partners

Abstract

Mitochondrial Complex II (CII) activity supports multiple types of respiration and connects metabolic state with succinate signaling. It is therefore unsurprising that CII activity has been linked to biological phenomena that require crosstalk between metabolism and signaling, including neurodegeneration, metabolic shifts in cancer, immune activation, and ischemia reperfusion injury. In each of these processes, CII activity may be regulated at the level of assembly, with the disassembled SDHA subunit stored as metastable species with one or more assembly factors. However, both the exact molecular nature of these species and the mechanism of release of SDHA from these metastable species is unclear. In this work, we identified that transfer of SDHA into CII proceeds through at least three metastable intermediates, we assigned the sequence of appearance of these intermediates, and we determined structures for two of these. A complex between SDHA and one of its assembly factors, succinate dehydrogenase assembly factor 2 (SDHAF2), is likely the main alternative species that accumulates in cells. A second assembly factor, succinate dehydrogenase assembly factor 4 (SDHAF4), must displace SDHAF2 for assembly to proceed. Changes in intrinsic disorder in both SDHAF2 and SDHAF4 are required to release SDHA from the metastable complexes. Evaluation of unrelated biological complexes suggests that changes in disorder may guide post-folding biogenesis during both self-assembly and assisted assembly.

An evolving view of complex II—noncanonical complexes, megacomplexes, respiration, signaling, and beyond

“…Of these, SDHAF2 has homologs in all kingdoms of life, with bacterial homologs termed SdhE and yeast homologs termed Sdh5. In contrast, SDHAF4 is pan-eukaryotic with a limited number of homologs in bacterial α- and γ-proteobacteria that are not commonly used as model systems ( 140 ). As a result of the low number of bacterial models for SDHAF1, SDHAF3, and SDHAF4, studies of these complex II assembly factors combined work other models, particularly yeast, due to the ease of genetics.…”

“…3 A ) is highly stable, long-lived, and appears to accumulate in cancer cell lines ( 21 ). The role of SDHAF4 (yeast Sdh8) is more cryptic, but it releases SDHAF2 from the SDHA–SDHAF2 complex ( 140 ), which is a requirement for the assembly of functional complex II ( 136 , 137 , 148 ). SDHAF1 and SDHAF3 are proposed to act as chaperones for Fe-S assembly within the SDHB subunit ( 138 , 139 ).…”

“…A , complex II-low is a heterogeneous mixture of species that contain SDHA, SDHAF2, and SDHAF4 that runs as a broad band on a native gel ( 21 ). The predominant species is likely the stable SDHA-AF2 complex (PDB 6VAX , ( 140 , 142 )). However, it cannot be excluded that complex II-low contains SDHA-AF2-AF4 (PDB 8DYD ( 140 )) or SDHA-AF4 (PDB 8DYE ( 140 )).…”

“…The predominant species is likely the stable SDHA-AF2 complex (PDB 6VAX , ( 140 , 142 )). However, it cannot be excluded that complex II-low contains SDHA-AF2-AF4 (PDB 8DYD ( 140 )) or SDHA-AF4 (PDB 8DYE ( 140 )). B , the SDHB subunit forms complexes with TRAP1, which may have implications for regulating cancer metabolism (PDB 7KCM ( 133 )).…”